A number of different methods have been used in the detection of metals and metallic minerals in sediments on the sea floor. The methods include seismic, magnetic and resistivity profiling techniques. Although these prior art methods are well developed, for a further discussion of some of these methods, reference is made, for example, to U.S. Pat. No. H1490 (Thompson, et al), U.S. Pat. No. 5,430,389 (Wynn, et al.), U.S. Pat. No. 4,617,518 (Srnka), U.S. Pat. No. 3,562,633 (Swain), U.S. Pat. No. 3,182,250 (Mayes), U.S. Pat. No. 3,052,836 (Postma), U.S. Pat. No. 2,872,638 (Jones), U.S. Pat. No. 2,839,721 (Dewitte) and U.S. Pat. No. 2,531,088 (Thompson). Briefly considering some of these patents, the Wynn, et al. patent discloses a sensor for detecting magnetic fields and current dipoles of materials and objects buried in sediment using a towed coil, while the Srnka patent discloses a towed system including an electric dipole current source which emits an alternating current and a plurality electric dipole detectors which measure potential differences between the detectors. The remaining patents generally relate to ocean floor logging methods using towed detectors which measure and log either changing potential differences or resistivity along the path of the towed detectors.
As discussed below, the method of the invention employs induced polarization (IP) in the identification process. The IP effect is a current-induced electrical response detected as a delayed voltage in certain minerals and, as described below, the method has been used for some time in the detection of these minerals in the ground. One manifestation of the response is that the voltage on an array of detectors or receivers lags the primary or inducing voltage (produced by a transmitter) by a finite amount of time. This is usually expressed as a phase-shift, i.e., a slight shift of the wave-cycle between the transmitter and receiver, and is usually reported in units of milliradians, where one duty cycle of the transmitter is 2.pi. radians.
For many decades it has been known that pyrite, most other metallic-luster minerals, and certain clays give rise to an IP effect. Geophysicists have taken advantage of this fact to discover and map large disseminated sulfide bodies (primarily copper and molybdenum) since the 1950's. The phenomenon is based on a complex double-layer interaction of ions in the electrolyte (the ground water) and the individual mineral surfaces. Because of this, IP is more sensitive to surface area than to volume and finely disseminated minerals make the best targets. An IP survey typically gathers both resistivity information, which is generally a measure of the porosity of the substrate, and polarization information, which is a measure of the reactivity of certain minerals (i.e., those described above) disseminated throughout the subsurface. Computer modeling can then be used to arrive at models that best fit the observed data acquired on the surface, with the purpose of providing a true map of the three-dimensional nature of the subsurface. The use of two physical characteristics (resistivity and polarization information) instead of just one makes the interpretation much more reliable.
There are, of course, a number of patents relating to the use of induced polarization in the detection of minerals and other materials and among these are the following: U.S. Pat. No. 5,671,136 (Willhoit), U.S. Pat. No. 4,467,642 (Givens), U.S. Pat. No. 4,041,372 (Miller, et al.), U.S. Pat. No. 3,984,759 (St. Amant, et al.).